Technical Field
The present invention relates to a machine element, in particular a motor spindle or an electromechanical drive unit, with integrated, internal cooling system and to a closed cooling circuit containing a machine element with an integrated, internal cooling system.
Discussion
Great importance is today attached to the cooling of machine components in machine tools. Modern machine tools have high precision and, in mould making, in particular, the requirements placed upon the machine tool and the therein installed motor spindles—above all, on the main spindle of a milling head—have risen massively in recent years. The quality of the surface machining of a workpiece is substantially impaired by vibrations of the working spindle, the machining process, and by the temperature behaviour of the motor spindle and of the machine frame. In respect of the vibrations generated, inter alia, by the material-removing machining of the workpiece, a great deal has been undertaken in recent years, so that the high performance spindles nowadays meet the requirements.
In terms of thermal behaviour, on the other hand, a limit has been reached with the known cooling options. An ideally conditioned spindle exhibits a constant temperature and a homogeneous temperature distribution within the spindle and across all operating states. The reality looks different, however: spindles have during operation localized heat sources, which lead to a non-homogeneous heat input. Typical heat sources within a spindle are in this context the bearings (friction) and the motor (Cu, Fe, supplementary and harmonic losses). These warm the real motor spindle unevenly, whereby a non-homogeneous temperature distribution is obtained within the spindle. The temperature here varies both in the circumferential direction (so-called polar temperature distribution) and in the axial direction.
The inhomogeneity of the temperature distribution can in motor spindles familiarly be reduced with a cooling system, which system should also ensure a constant temperature at different load. However, known cooling systems, above all in respect of precision spindles, are not capable of keeping the temperature differences sufficiently low. This limited capability of known cooling systems currently poses a major problem.
The machine elements installed in machines, in particular machine tools, are in certain cases cooled or thermally stabilized with cooling systems which have a closed cooling circuit. As already mentioned, the cooling is effected for various reasons:                The machine element produces waste heat which must necessarily be removed in order to secure the working of the element. Otherwise the element would directly fail due to overheating, or the efficiency, useful power or working life would be severely reduced.        The machine element must also, however, be thermally stabilized so as to be able to correctly fulfil its function—for example the machining of a workpiece. This is particularly true of precision-relevant components of a spindle, which are generally made of steel and, due to their thermal expansion upon changes in temperature, change their dimensions.        
Since machine elements of a machine tool must generally be warmed to operating temperature only in a start-up phase, during running operation the cooling function remains dominant. In the following, for simplification purposes, reference is made to a cooling circuit, even if this serves not only for the pure unregulated cooling, but also for the temperature stabilization of the machine element (i.e. the cooling capacity is adapted to the quantity of heat to be removed). Furthermore, such a cooling circuit also influences the temperature distribution within a machine, that is to say between the various machine elements (for example milling head, bearings of the milling head, and machine frame in the region of the milling head). This is not examined in detail below, but simplistically it can be said that the best state is ensured if all machine elements and all subcomponents] within a machine tool have the same (operating) temperature. This temperature is referred to below as the target temperature. The solutions which are described below can, where necessary, be analogously adapted for other cases. This target temperature lies in the region of the room temperature, often a few degrees K higher (for instance, 24° C. are customary), in order that the convection with the ambient air tends to ensure a low heat flow into the environment, and not the other way round, that the machine cooling cools the room. For simplification purposes, it is assumed below that the target temperature lies above the ambient temperature, though, in the reverse case, the statements can be adapted analogously.
As is known, a closed cooling circuit comprises at least one heat source, one heat sink, and a pipe system in which a cooling medium circulates between the heat source and the heat sink. This circulation is generally enforced by a pump. As a suitable cooling medium, a water-based coolant is often used, since it has a low viscosity and a high specific heat capacity. The flow rate of the cooling medium is limited, however, by the design of the heat source (for example size of the spindle). Above a certain range, an increase in the heat removal is therefore only possible by increasing the heat capacity of the cooling medium. A low viscosity here additionally facilitates the circulation. A low flow rate and low viscosity of the cooling medium is advantageous, moreover, because the dimensioning of the pipes and of the pump thereby turns out to be smaller and, in the heat transfer at the heat sink or the heat source, a smaller necessary contact surface is required. By way of example, water is often simplistically specified as the cooling medium, even if, according to application and for specific reasons, this can also be a different liquid.
FIG. 1 shows in schematic representation the working method of a spindle cooling system with closed cooling circuit 24. The motor spindle 6 (heat source) to be cooled is cooled by means of a cooling medium or coolant 4 (for example water), which, driven by a coolant pump 5, flows through cooling lines 7 into the internal cooling system 8—in this case cooling lines arranged helically around the circumference of the motor spindle 6. The warmed cooling medium 4 which exits the built-in spindle cooling apparatus or internal cooling system 8 flows, for its part, again via cooling lines 7 back into a reservoir (heat sink) 9, where the heat is again extracted from the cooling medium 4. This heat extraction is effected in the reservoir of the heat sink 9 by a cooling compressor 1, which is regulated by means of a temperature monitor 2, which cools the cooling medium, for instance, to 24° C. The cooling compressor 1 itself can here have a lower temperature than 24° C. In the represented closed cooling circuit 24 of FIG. 1, a flow monitor with signalling contact 3 is additionally built in.
FIG. 2 shows, furthermore, how the cooling lines of an internal cooling system 8 could actually be arranged in a motor spindle.
It is commonly known to provide cooling apparatuses in machine tools. Thus, EP1252970A1, for instance, discloses how, in a machine tool having a closed hood, with the incorporation of cooling circuits and heat convection by the air, the fundamental elements of the machine can advantageously be brought closer to a reference temperature.
Publication EP 376 178 A1 sets out how a motor spindle in a machine tool is designed with a cooling system with gaseous cooling medium in order to be able obtain sufficient cooling. The topic of temperature differences between the forward circuit and the return circuit is not raised in the document.
EP 1 927 431 A1 shows an advantageous design of the heat sink for a spindle cooling system with which the forward circuit temperature of the cooling medium for the spindle can be regulated within narrow limits. From this document can be seen the high level of complexity which is associated with temperature stabilization in a conventional cooling system.
In cooling systems constructed in this way, the naturally limited heat capacity of the cooling medium limits the heat removal and further functions of the cooling system for several reasons:
In the first place, cooling mediums react “sensitively”, i.e. the absorbed heat increases the temperature of the medium in inverse proportion to the heat capacity of the medium. When cooling medium enters the heat source, the temperature is inevitably lower than when it exits. It is therefore not possible to stabilize with a cooling circuit a plurality of heat sources in series (cf. FIG. 2) at the same temperature, especially not if they behave in a time-variable manner. A spindle or a machine element frequently, however, has more than one heat source (for example on the front or rear bearing or on the windings in the middle region of the spindle), wherein ideally all regions of the machine element should be stabilized at the same temperature. This drawback can be alleviated with a parallel connection, but this gives rise to further problems, such as the steering of the flow through the various parallel branches of the cooling network. Irrespective of parallel or serial connection of the cooling circuits, the number of existing heat sources, or the mass flow of the guided cooling medium: A machine element can never by these measures be kept isothermal in terms of time and location—i.e. the element has the same and constant temperature everywhere.
In the second place, the heat flow from the heat source to the cooling medium is dependent on the temperature difference. If the temperature of the source rises due to a higher generated heat quantity (for example due to higher motor output), then, as a consequence of the local heat absorption, the temperature of the cooling medium also rises (the cooling medium thus reacts sensitively). As a result of this temperature rise, the temperature difference between the heat source and the cooling medium declines and the heat flow is thereby lessened, the cooling capacity consequently has a tendency to decline, and this in cases and at locations in which specifically more cooling capacity would be demanded.
In the third place, the cooling capacity is substantially dependent on the flow of the cooling medium. If the flow is increased, the necessary pump pressure rises in respect of a given pipe cross section. As a consequence thereof, the pump output has to be increased and the increased pump pressure inevitably warms the cooling medium also. This waste heat must be removed from the cooling medium itself and thus already lessens the cooling capacity at the actual heat source. The available cooling capacity can therefore be increased only underproportionally by raising of the flow.
In the fourth place, with increased flow there is the danger of formation of a turbulent flow in the cooling medium, which increases the necessary pump pressure and thus still further increases the pump output and leads to the described reduction in the available cooling capacity.
In the fifth place, the pipe diameter and the shape of the cross section can often not be freely chosen in practice. The machine elements in question must meet a variety of demands and their conceptual design constitutes the best possible compromise for the optimal satisfaction of these demands. The available space for the cooling circuit is limited and, as a result of the complexity of the elements, is subject to various, including above all geometric, restrictions (see cooling lines in FIG. 2).
In the sixth place, the regulation of the temperature of the cooling medium poses a relevant difficulty. A narrow tolerance in relation to the target temperature can only be ensured with complex assemblies, as well as a sensor system, hardware and software for the regulation. Usually, such cooling systems are operated with a so-called two-point controller. This means that the heat sink cools as soon as the upper control point is reached, so that the cooling medium, upon leaving the heat sink, periodically fluctuates in temperature between the lower and the upper control point. For instance, from machining spindles for precision machining, it is known that this type of fluctuation, even if it amounts to just a few degrees Kelvin or even to less than 1 K, already has an adverse effect during use.
In the seventh place, the machine elements cool off during stoppages and, in the case of precision machining, for instance, following resumption of the operation, a warm-up phase is necessary in order to restore a thermally stable operating state.
Fully divorced from the preceding statements, in other technical fields so-called Phase Change Materials (PCMs) are known for their heat absorbency. PCMs are materials which at a defined temperature perform a phase transition and, in so doing, either release or absorb a large quantity of heat. In the midst of the phase transition—for example upon the change of state from solid to liquid—the temperature is not altered by the inflow or outflow of heat. Outwardly, the impression is given that the specific heat capacity of PCM-containing liquids is substantially higher than with conventional cooling mediums. The investigation of PCMs was conducted, above all, in order to acquire a higher storage density for heat, in particular for the storage of solar heat. In addition, PCMs are used in so-called latent heat stores, inter alia in building technology, in order to increase the thermal inertia of buildings and to reduce power peaks. The levelling of periodically occurring temperature fluctuations by means of latent heat stores is also known.
Document EP 2 375 483 A2 discloses, for its part, the use of PCMs as a suspension or emulsion in a cooling medium on a water-free base. This PCM-containing cooling medium is applied in fuel cells, with use being made of the high heat capacity of the PCMs. As a PCM is described anorganic salt in a water-free liquid. Although EP 2 375 483 A2 discloses the use of a dispersion having PCMs as the cooling medium, the document gives no hints or suggestions as to how the stabilization of a fuel cell at a target temperature could be carried out.
Document EP 0 987 799 A2 describes a passive cooling system for the short-term cooling and thermal stabilization of a solid-state laser. The disclosed cooling system uses a solid PCM cooling body, comparable with cooling elements of standard coolboxes. A cooling system having a solid PCM cooling body is functional, however, only for a few minutes and, according to this document, is usable for instance, for the final route guidance of a guided missile. For a continuous operation of the described cooling system, EP 0 987 799 A2 proposes combining the solid-state PCM with a heat exchanger which operates with a cooling liquid. The phase state of the PCM cooling body, or the solid to liquid component, can thereby be favourably influenced. Document EP 0 987 799 A2 also proposes the creation of cooling bodies from various PCM materials.
From Document U.S. Pat. No. 5,141,079, it is known to use cooling lubricants to cool the machining site between the tool and the workpiece on machine tools which contain microencapsulated PCMs as a component. The described cooling lubricants lubricate and, at the same time, cool the tool and the workpiece at the machining site very effectively. This thanks to the heat capacity of the PCMs contained in the cooling liquid. The cooling lubricant here works in an open or external cooling circuit and serves merely to cool the machining site of the workpiece or merely to cool the tip of the milling or turning tool.